Annotate arg MaterialButton.setIcon(@nullable)

PiperOrigin-RevId: 245728195

Author: Material Design Team

lib/java/com/google/android/material/button/MaterialButton.java

@@ -541,7 +541,7 @@ public int getIconSize() {
    * @see #setIconResource(int)
    * @see #getIcon()
    */
-  public void setIcon(Drawable icon) {
+  public void setIcon(@Nullable Drawable icon) {
     if (this.icon != icon) {
       this.icon = icon;
       updateIcon();